Toners used in electrophotography, electrostatic recording, or electrostatic printing are, in a developing step, deposited temporarily on image bearers (e.g., electrostatic latent image bearers) on which electrostatic charge images have been formed. Next, in a transfer step, the thus-deposited toners are transferred from the electrostatic latent image bearers onto transfer media (e.g., transfer paper). Then, the thus-transferred toners are fixed on the media in a fixing step. A commonly used method for fixing the toners is a method in which the toners are heat-melted by being brought into contact with a heated roll or a heated belt to fix the toners. This is because this method is excellent in thermal efficiency. However, the method is problematic in that offset is likely to occur. The offset is a phenomenon in which melted toners are melt-adhered to the heated roll or the heated belt.
In order to prevent the offset, release agents (e.g., waxes) may be added to toners themselves. In the above method, the release agents rapidly melt when the toners pass through the heated roll member or the heated belt member, to be exposed on surfaces of toner particles. Thus, the release agents prevent the toners from melt-adhering to fixing members. The release agents have effects on offset at low fixing temperatures (cold offset) and offset at high fixing temperatures (hot offset).
In the case where the release agents are disposed adjacent to surfaces of toner particles in order to facilitate exposure of the release agents from the toner particles, the offset is prevented but other problems are caused. For example, the toner particles are likely to melt-adhere to a carrier or a photoconductor via the release agents during stirring in a developing device. As a result, the toner particles adhere to the carrier or the photoconductor in a crushed form to increase the possibility of decreasing a charging amount of the toner particles. Therefore, it is necessary that the release agents are protected by existing inside the toner particles during stirring or storage, but are exposed on the surface effectively in a short time while the toner particles pass through the fixing member in the fixing step, to exert releasability from the fixing member.
In order to address the above problem, many proposals have been reported with regard to waxes which serve as the release agent and have specified dispersed particle diameters (see, Patent documents 1 and 2). The waxes are effective in maintaining toner granulation performance and preventing the offset at the same time. This effect results from the specified dispersed particle diameter.
However, when a wax is introduced into a toner in a dispersed state, the wax particle diameter typically must be smaller than the toner particle diameter. Therefore, it is very difficult to hold the wax having such a small diameter inside the toner without exposing adjacent to the surface of the toner.
The release agent can more effectively exert offset-resistance in a relatively large aggregate form than in a localized form as small domains in the toner. However, when an unnecessarily large amount of the release agent is added in order to enlarge the domains, toner strength as a whole is weakened to increase the possibility of being crushed. As a result, the toner is more likely to have a decreased charging amount or to deteriorate in background fog.
In particular, when a toner including a release agent is used for non-magnetic one-component developing, an excessive load is applied to the toner by a blade configured to regulate a toner-layer thickness, while the toner passes through the blade. Thus, the toner is crushed to adhere to the blade. This has been found to significantly deteriorate image quality. Therefore, the toner needs to have higher durability than a toner used for two-component developing.
The toner described in the Patent document 2 includes a release agent having a specified aspect ratio and a specified size. The toner is improved in low-temperature fixing ability, background fog, and chargeability. However, the toner is unsatisfactory in terms of exhibiting excellent offset resistance and achieving excellent chargeability and excellent durability through improvement of toner-particle strength.
There has been proposed a toner which is produced by discharging a toner composition liquid from discharging holes to make the toner composition liquid into the liquid droplets. The thus-produced toner is excellent in hot-offset resistance and background fog (see Patent document 3).
However, this technique also has a room for improvement in terms of exhibiting excellent offset resistance, and achieving excellent chargeability and excellent durability through improvement of toner-particle strength.